BUG: SIB MOV EAX,[EAX+EBP] Generates Incorrect Code

• Microsoft Macro Assembler for MS-DOS, versions 6.0, 6.0a, 6.0b, 6.1, 6.1a, and 6.11
• Microsoft Macro Assembler for OS/2, versions 6.0, 6.0a, and 6.0b

SYMPTOMS

An attempt to use scaled index base (SIB) indirect operands may fail. If EBP is used and the stack and data segments do not have the same base address, the generated code executes incorrectly. In addition, if EBP is used as the index register, it is coded as the base register, which generates an additional byte in the instruction encoding.

CAUSE

When the code does not use a scaling factor, the Microsoft Macro Assembler (MASM) may reverse the base and index registers. When this occurs, MASM incorrectly treats the first register as the index and the second register as the base in an SIB indirect operand.

If the code uses EBP or ESP as the base register, the indirect operand should access memory through the stack segment. When the code uses any other register as the base register, the indirect operand should access memory through the data segment. Because the assembler reverses the base and index registers, it uses the index register to determine the segment through which to access memory. However, the ESP register can be used only as a base register. Therefore, the problem occurs only when the code uses the EBP register.

RESOLUTION

To work around this problem, use a scaling factor to explicitly indicate which register is the index register. The sample below demonstrates this procedure.

STATUS

Microsoft has confirmed this to be a problem in MASM versions 6.0, 6.0a, 6.0b, 6.1, 6.1a, and 6.11. We are researching this problem and will post new information here in the Microsoft Knowledge Base as it becomes available.

MORE INFORMATION

When the code does not use a scaling factor, MASM should treat the first register as the base and the second register as the index in an SIB indirect operand. Note that the code must use the .386 or .486 directives to use an SIB indirect operand. One of these directives must be present to demonstrate the problem above.

Sample Code

; Assemble options needed: /Zi ; Use Codeview to see the instructions generates by the assembler.

.MODEL small, C

.386                      ; .386 must be used to show the problem

.DATA

.CODE

    .startup
    mov eax, [ebp+edi]    ;Incorrect
                          ;MOV AX,WORD PTR DS:[EDI+EBP]
    mov eax, [esi+ebp]    ;Incorrect
                          ;MOV AX,WORD PTR SS:[EBP+ESI]
    mov eax, [ebp][edi]   ;Incorrect
                          ;MOV AX,WORD PTR DS:[EDI+EBP]
    mov eax, [esi][ebp]   ;Incorrect
                          ;MOV AX,WORD PTR SS:[EBP+ESI]
    mov eax, [ebp+1*edi]  ;Correct
                          ;MOV AX,WORD PTR SS:[EBP+EDI]
    mov eax, [esi+1*ebp]  ;Correct
                          ;MOV AX,WORD PTR DS:[ESI+EBP]
    .exit 0